The protection of transmission media in building distribution systems is important to the integrity of the transmission media. Such transmission media include, for example, optical fiber and coaxial cable. As is well known, coaxial cable includes a center metallic conductor enclosed in a layer of dielectric material, a tubular outer metallic conductor and a plastic jacket. As also is well known, copper transmission media such as coaxial cable were the mainstay of the communications transmission market until the 1980's. During that decade, the use of optical fiber advanced at an incredible pace. Optical fiber which offers greater bandwidths and provides greater immunity to electromagnetic noise and interference to ensure error-free transmission of information is destined to experience widespread use in premises distribution systems.
Customers, users, and sales personnel share the concern of the possibility of damaging installed fiber, especially where copper conductors and optical fiber are integrated in common transmission media systems. For example, in data link applications, fibers are routed into data cabinets which house electronic equipment. In most cases, it has been the practice to remove a protective outer sheath system from a bundle of fibers at or near the point of entry into the cabinet. After connector plugs are installed on each fiber, they may be run alongside power, twisted pair, or coaxial cables, thereby placing them in a vulnerable environment. Activities such as equipment change-out, testing, and cable rearrangements are normal occurrences.
Also to be avoided is damage to optical fibers which are routed under carpets from wiring closets to workstations. Undercarpet cables, which transmit voice and data communications, are being used increasingly today in buildings. These are needed to extend transmission medium service from ceiling plenums or risers to various locations on a building floor to service office equipment. Obviously, drops from an overhead plenum could be made directly at points where connections must be made, but such drops are unsightly. Hence, from an aesthetic point of view, it becomes important to be able to provide cables which may be routed under an office carpet.
Buffered optical fibers have been used in such environments. A buffered optical fiber is a coated optical fiber which includes an additional plastic cover, the material of which generally has been polyvinyl chloride (PVC). However, in these kinds of environments, there has developed a feeling of insecurity about the protection offered by the buffer layer which surrounds the optical fiber.
Another concern which has developed is the comparatively high cost involved in providing post-construction undercarpet cables to off-wall workstations within buildings. Although the cost for undercarpet cable may be higher than costs for conventional cables which are installed under carpet, the installation costs for the latter bring the total installed cost to be higher than the former. Also, cables specially designed for undercarpet use are ideal for campus type environments and for renovations.
Requirements for a cable that can be used under carpet are somewhat stringent. Of course, the cable must include provisions for protecting mechanically the transmission media included therein. This is particularly important when the transmission media is optical fiber. Also, the layout of undercarpet cable on any given building floor may involve tortuous routes wherein the cable must assume a curved configuration. Any cable which overcomes the foregoing problems must be suitable for the inclusion of optical fiber as well as copper media. Optical fiber which is relatively fragile must be protected from abuse either in the gross sense which, for example, involves macrobending, or in the minute sense of microbends. Further of importance is the ability to customize in the field cables for such uses. This capability would expedite installations and add a new dimension to undercarpet communications wiring systems.
Desirably, cables which may be suitable for use under carpets also should be capable of being used in adjacent cabinetry, for example. Therein, cables may be needed to extend service from incoming lines to outgoing lines which extend under a carpet.
One way used to route electrical conductors is by the use of a copper ribbon cable which has a plurality of spaced parallel electrical conductors disposed within a thin, flexible layer of insulation and which is disposed under carpeting in, for example, office areas. A change in direction in such a flat cable is achieved by sharply folding the cable upon itself so that stacked layers of the cable result at the bend. Although such a method of changing direction can be used with a ribbon cable having spaced copper wires which can undergo a 90 or 180 degree bend, this method of changing direction is not suitable for a cable assembly which includes optical fiber or coaxial cable. Sharp bending of optical fiber will result in light attenuation and sharp folding of optical fiber will cause breaks. Folding of a coaxial cable will mechanically damage the shield, displace the dielectric between the inner and outer conductors, and cause a change in the impedance characteristics of the cable. Also, folding of a cable doubles its thickness, which could make the presence of the cable under carpeting more noticeable. Of course, bending of a flat cable in the plane of the cable results in no appreciable increase in cable thickness. However, copper ribbon cable is not capable of being bent in the flat plane of the cable without curling unless the copper media therein can move independently of the ribbon material.
Other approaches include the use of preformed, flat conduit lengths which are connected together in a longitudinal direction to form a track. Each segment contains grooves into which optical fiber may be placed. Systems such as these appear to be used mainly for permanent installations, inasmuch as any rearrangement would be difficult and labor intensive. Another approach is to use fully-sheathed flat undercarpet fiber cables, which also can be installed in a post-construction environment. These are ideal for straight line runs; however, any change in direction may require the cutting away of portions of the sheath and the addition of hardware to achieve bends and turns. Still another alternative is to cut channels into a floor and to install conduit, a very costly choice.
One recently proposed coaxial cable assembly for use under carpeting includes a jacket of polyvinyl chloride (PVC) having a central portion, which holds a small coaxial cable, and side portions each having a stress-bearing plastic member. The stress-bearing members, which are relatively inelastic, are independently longitudinally movable in the jacket. Bending of the cable assembly causes the member at the inside of the bend to extend beyond the jacket while the member at the outside of the bend is drawn inside the jacket.
A cable of U.S. Pat. No. 4,665,280 provides a fixture which provides both support and protection for elongated members of an undercarpet cable throughout the length of a turn. The fixture is a formed rigid member defining at least two tracks between an inlet and an outlet spaced apart in a plane and angularly offset with respect to each other. One of the tracks defines ar arcuate path. Each subsequent track is spaced radially inwardly from the arcuate track and defines a curvilinear path of equal length to the length of the arcuate outer path.
Some cables which have been used for undercarpet situations for transmitting light or power have included a pair of sloped surface flange portions on the marginal sides of a central web which holds one or more optical fibers, strengthening members and/or electrical conductors. A typical example of such cable can be found in U.S. Pat. No. 4,419,538. See also U.S. Pat. No. 4,801,764. In bending this type of cable, the inner sloped flange must be compressed while the outer sloped flange is stretched and the elongated members are relatively longitudinally displaced. It should be appreciated that the bending of a flat object in the plane of the object is not an easy task.
A. cable assembly disclosed in U.S. Pat. No. 4,815,814 includes at least one resilient buffer tube and at least one optical conductor inside the tube and movable therein. At least one strength member and a plastic jacket formed about the tube and the strength member are included in the assembly. The jacket may have substantially flat, parallel top and bottom surfaces with the tube and strength member being disposed in the jacket substantially midway between these surfaces. The tube and the strength member are spaced and their respective axes are positioned in a plane which is generally parallel to the bottom surface. Also disclosed is a generally flat fiber optic cable assembly manufactured with a right angle turn section.
The prior art also includes an undercarpet cable which has a so-called zip design which allows breakout of a duplex fiber optic center subcable portion of the cable. There may be other alternatives, but those described hereinbefore are the most prevalent and illustrate some of the problems associated with current offerings of undercarpet fiber cable.
What is sought after is an undercarpet cable which accommodate one or more transmission media. The sought after cable desirably is thin and relatively flat, and limits bending in the plane of the cable assembly so that the transmission media will function properly and will be protected from mechanical damage. The cable should include provisions for facilitating breakout of portions of the transmission media from remaining portions. Also, the desired cable should be unobtrusive when installed under carpeting and should be capable of supporting normal loads without functional or mechanical damage to the transmission media. Furthermore, the cable should be reliable in use, have long service life, be lightweight and be relatively easy and economical to manufacture.
What is needed and what seemingly is not yet available in the prior art is an undercarpet cable system which is robust and which is easily routed in expected paths within an office. The sought-after cable would be one which desirably may be custom fitted to particular fiber sizes depending, for example, on the number of workstations to be serviced by the cable.